Metal capacitors are important components that may be used in transceivers such as in a high speed serial interface. Metal capacitors are also used in other analog circuits such as high pass and low pass filters and other components. Generally, a high quality factor, Q, is desirable for high performance. The quality factor is defined as energy stored in a capacitor divided by the energy dissipated by parasitic resistance.
Two types of structures have been commonly used to implement metal capacitors. The first type of structure included the use of long parallel metal lines on each layer of a semiconductor device, where the metal lines were also parallel between layers of the semiconductor device when multiple layers were used. Using long parallel metal lines biased at different potentials produced a high inductive effect on the metal capacitor during high frequency applications. This would result in fluctuations in the capacitance of the metal capacitor which was undesirable during high frequency applications.
The second type of structure is known as a rotative metal-oxide-metal (RTMOM) capacitor. Similar to the first structure, RTMOM capacitors utilized parallel metal lines on each layer of a semiconductor device. However, RTMOM capacitors used multiple layers where each layer positions its parallel metal lines perpendicular to the parallel metal lines on adjacent layers to form a woven line appearance. The inter layer coupling for the woven line style of the RTMOM capacitors proved to be inefficient. Lower capacitance density resulted as a larger line space was required to implement the RTMOM capacitors due to reliability concern under applications where voltage drops higher than nominal.